1. Field of the Invention
The present invention relates to an improved process for the manufacture of siloxane-oxyalkylene copolymers. More particularly, the present invention relates to a process for the continuous production of silicone-containing copolymers with polyalkoxy substituent chains, and to the products produced by the process
2. Description of Related Art
Silicone-containing copolymers were identified as stabilizers for polyurethane foam as early as 1958 by Bailey in U.S. Pat. No 2,834,748, and have been the subject of numerous subsequent patents. They also serve in many other applications, most frequently as surface-tension lowering agents in agricultural adjuvants (see Stevens, P. G, Pesticide Science, 1993, 38:103-122), but also as additives for coatings applications (see Fink, F, Journal of Coating Technology, 62, No. 791, December 1990), antifoams (International published patent application PCT/US94/06804), and emulsifiers (U.S. Pat. Nos. 4,782,095 and 4,801,447). The total volume of such copolymers manufactured worldwide each year has been estimated to be in excess of 100 million pounds.
The reaction of hydrosilatable olefins, such as allyl-terminated polyalkyleneoxides or 1-octene, with hydrosiloxanes such as poly(dimethyl)(methylhydrogen) siloxanes in the presence of an appropriate catalyst is known. Likewise, the reaction of hydrosilatable olefins, such as allyl chloride or 1-octene, with hydridosilanes such as trimethoxysilane, in the presence of an appropriate catalyst, is known.
U.S. Pat. No. 5,559,264 discloses a method for preparing chloroalkylalkoxysilanes by reacting an allylic chloride with a small molar excess of a hydromethoxysilane in the presence of a ruthenium catalyst and preferably in the substantial absence of an inert solvent.
U.S. Pat. No 5,986,022 discloses a continuous process for producing silicone copolymers using a series of at least one stirred-tank reactor, the last of which reactors in said series has crude product stream feeding into at least one plug flow reactor, wherein this crude product stream is sufficiently homogeneous that this stream which undergoes further reaction in the plug flow reactor does not undergo phase separation.
U.S. Pat. No. 6,015,920 discloses a process for hydrosilation reactions between olefins and hydrosilanes or hydrosiloxanes wherein a portion of the reactor output is recycled continuously to the reactor.
U.S. Pat. No. 6,291,622 discloses a continuous process for preparing organomodified polysiloxanes by the transition metal-catalyzed addition of polysiloxanes containing SiH groups onto substances containing C—C multiple bonds, in particular a process for the continuous hydrosilylation of substances containing C—C multiple bonds, which comprises introducing the reactants, in the presence or absence of a homogeneous or heterogeneous catalyst, into a loop-like, heatable and coolable reaction circuit which has a static mixing element and/or a dynamic mixing element to mix starting materials and product formed, leaving the reaction mixture circulating in the reaction circuit until a predetermined degree of conversion has been reached and subsequently transferring the reaction mixture still containing starting materials to a tube reactor to complete the reaction and taking it off via a receiver. In addition, a suitable industrial apparatus for carrying out the process is described.
U.S. Pat. No. 6,410,772 discloses a continuous method for performing a hydrosilylation reaction comprising effecting a hydrosilylation reaction between a liquid organosilicon compound (A) having in each molecule at least one silicon-bonded hydrogen atom and a liquid organosilicon compound (B) having in each molecule at least one aliphatic unsaturated bond in the presence of a platinum catalyst (C) continuously in a tubular reactor equipped with a stirring and plug-flow maintaining apparatus located within the reactor.
German Offenlegungschrift 196-32157 A1, discloses a process for the continuous production of organosilicon compounds of the 3-halopropylorganosilane type with the general structure:R3H3-a-bXaSi CH2CH2CH2Y                 where R stands for CH3, C2H5,, OCH3, O C2H5, or OC3H7;        Y stands for F, Cl, Br, or I; and where        a and be each stand for one of the numbers 0, 1, 2, or 3, the sum of a+b being equal to 1, 2, or 3. The formation of by-products is suppressed by preventing the educts from reacting completely, i.e., by adjusting the conversion to about 10-80% based on the total weight of the deficit component.        
U.S. Pat. No. 6,593,436 discloses a continuous process for the manufacture of silicone copolymer utilizing at least one static mixing plug flow reactor, and optionally two static mixing plug flow reactors in series or parallel. Silicone copolymers produced in accordance thereof are substantially free of unreacted hydrogen siloxane starting material and may be used without further purification. The static mixing plug flow reactor contains static mixing elements capable of creating eddies and vortices of sufficient intensity that a biphasic liquid mixture, such as hydrogen siloxane fluid and a polyether olefinic reactant, undergoes shearing of the droplets of each material so that one phase disperses into another to provide intimate contact between the two phases to allow the reaction to proceed until phase separation is no longer chemically possible.
There is interest in finding improved modes of carrying out the hydrosilation reaction. Improvements are elusive because of the variety of byproducts that typically are formed, their properties, and the need to control their formation and to remove those that do form from the desired siloxane copolymer product In addition, the hydrosilation reaction itself is sensitive to a number of conditions such that it can become necessary to balance competing effects and to accept non-optimum results.
Certain process schemes for carrying out the hydrosilation reaction, while effective, pose drawbacks. For instance, typical batchwise operation produces a crude product containing the desired hydrosilated siloxane copolymer in mixture with byproducts, reaction solvents, and one or more unreacted reactants. This crude product needs to be treated to recover the desired product in a subsequent stage, and it needs to be stored until it is passed to that stage. This storage, even temporary, poses a risk of degrading the product, as well as a risk of the crude product undergoing cross-contamination with other products. Also, storing crude product within the reaction scheme represents an accumulated inventory that raises the overall cost of the process
Further, in a continuous process operation as described in U.S. Pat. No. 5,986,022, multiple stirred tank reactors are required prior to the use of a non-agitated plug flow reactor, otherwise phase separation of the reactants is likely to occur and will cause potential performance problems in the product. And although beneficial in certain applications, slightly different molecular weight distributions of copolymer products are obtained when compared to batchwise processing.
In the discussion that follows, the term “silicone-containing copolymer” is used to refer to the generic chemical entity obtained by the combination of a chemical entity containing the methyl siloxy moiety with at least one other chemical entity, such as with a polyether, with an alkyl olefin, or with a compound containing an olefinically unsaturated group and substituted with another chemical functionality; or with a combination of such entities. Thus, a terpolymer, for example, a polymeric entity containing dimethylsiloxy groups, polyether groups, and alkylmethylsiloxy groups, would be included in this definition of copolymer. Also, a dialkyl, tetramethyldisiloxane would be included in this definition, as would be an α,ω-[bis(polyoxyalkylene)propyl]polydimethylsiloxane.
The efficient manufacture of such copolymers is desired for two primary reasons: 1) lower cost, and 2) less waste. Although the second factor inherently influences the first, the relative significance on cost may be low; but the impact of waste on the environment, and consequently on the waste-treatment facilities that must be installed to prevent the copolymer from unintentionally reaching the environment, is large. Hence, a method or process of manufacture that is inherently more efficient is of considerable utility. If, in addition, the equipment needed for that method or process is less costly to construct, such method or process will be attractive to manufacturers.
The present invention fulfills this need by means of a continuous process. Continuous systems are much smaller than batch reactor systems and are thus less costly. But more importantly from an operating perspective, they contain much less product, and are thus much easier to clean. They thus generate less waste, if cleaning is implemented between two different products, and less material is lost from equipment “holdup”, so overall efficiency is higher. From an operating perspective, they are also more “controllable”, in the sense that the extent or degree of reaction is primarily determined by the reactor or equipment design, as opposed to a batch reactor system, wherein the extent or degree of reaction is primarily determined by elapsed time, which factor can be enormously influenced by a multitude of variables, such as purity of raw materials, temperature, material of construction, and others.
Chemical reactions may be conducted in a batch fashion, in a continuous fashion, or in hybrid fashion (partially batch or partially continuous). For example, in batch mode, the reactants necessary to prepare a silicone-containing copolymer of the type produced in the present invention are (1) a silicone methyl hydrogen fluid (hereinafter referred to as a hydrogen siloxane fluid); and (2) an olefinically-terminated polyether (hereinafter referred to as a polyether or an allyl-polyether) or another olefinically-terminated compound (hereinafter referred to as an olefin or an olefinic compound). The two components are mixed together, in appropriate amounts, with a noble metal catalyst added. A vigorous reaction ensues, and the olefin, by hydrosilation, becomes chemically attached to the silicone.
Because in most cases the hydrogen siloxane fluid and the polyether or olefin are immiscible, a compatibilizing agent is frequently used to facilitate reaction. This agent is often called a solvent, although it is not necessary to use it in sufficient quantity to totally dissolve both components. If the hydrogen siloxane fluid and polyether or olefin are sufficiently low in minor to trace components, the amount of “solvent” can be decreased (see U.S. Pat. Nos. 4,025,456 and 3,980,688), in some cases to zero. However, in those cases, good mixing becomes even more significant, so as to maximize the contact between the two (relatively) immiscible phases.
The reaction between the raw materials need not be conducted in a purely batch fashion. For example, if the reactivity of the hydrogen siloxane fluid is very high, the polyether or olefin may be charged to the reactor in its entirety, a fraction of the hydrogen siloxane fluid may be charged, the reaction catalyzed by adding a noble metal catalyst solution, and the remaining hydrogen siloxane fluid added subsequently and at such a rate, after the initial reaction exotherm has begun to subside, that the reaction is kept under control. This process is sometimes called semi-batch, or (incorrectly) semi-continuous. If both the hydrogen siloxane fluid and the polyether or olefin were added only in part initially, and then all components were added continuously after the reaction initiated, and added until the reactor were full, the reaction would be called (correctly) semi-continuous.
There are, in a general sense, two types of continuous reactors that are conceptually suitable for copolymer formation: continuous stirred tank reactors (known as CSTR's); and plug-flow reactors. A CSTR is simply a tank, usually vigorously agitated, into which the reactants and catalysts—all the components of a batch reaction—are fed continuously, and product is withdrawn continuously and at the same total rate as reactants are added. It is inherent, however, in this type of reactor, that not all of any of the reactants can be completely consumed uniformly. Although the system is vigorously agitated, fresh reactants, just momentarily previously introduced into the system, have a finite probability of exiting the reactor by withdrawal of the contents, along with old reactants that have spent much longer time in the tank, i.e., they have reacted, and, hence, have become crude product. A silicone-containing copolymer containing unreacted hydrogen siloxane fluid is well known in the art to be totally unsuitable for making certain polyurethane foams, for example, it affords low potency flexible/slab-stock foam and, at worst, can collapse flexible/slab-stock foam.
In the simplest version of a plug-flow reactor, all reactants are introduced into the front end of a pipe of sufficient length to ensure reaction completion. The pipe is usually maintained at the temperature of reaction, and reaction ensues along the length of the pipe. The length of the pipe is determined by the time necessary to cause the reaction to proceed to completion, i.e., at least one of the reactants has been completely consumed. The above described problem of unreacted hydrogen siloxane fluid exiting a CSTR reactor might be circumvented by the use of a plug flow reactor, were it not that without continued mixing, an immiscible hydrogen siloxane fluid and polyether will phase-separate very rapidly subsequent to initial mixing, thus causing the reaction to proceed more and more slowly. (In fact, the reaction ceases rapidly without ongoing agitation, and then fails to proceed, even upon renewed agitation, which effect is believed to be caused by gradual, irreversible deactivation of the noble metal catalyst.)
Thus, neither of the two standard continuous reactor systems alone are effective for the manufacture of silicone-polyether copolymers, or any other silicone-containing copolymer for which the reactants are immiscible, as taught in U.S. Pat. No. 5,986,022.
It might be argued that the compatibilizing agent, or “solvent”, referred to earlier, might serve as a means of maintaining phase compatibility in a plug flow reactor. However, the volume of solvent needed to achieve one phase is impracticably large, and any inherent advantage of a continuous reactor system is lost by virtue of the requisite size of equipment imposed by the large volume of solvent and by the added requirement for subsequently removing any volatile or otherwise interfering solvent from the copolymer to render it useful. In the absence of “solvent”, or in less than a fully compatibilizing quantity, and in a batch mode, and using standard hydrogen siloxane fluid and immiscible polyether or olefin reactants, the reaction frequently, and unpredictably, proceeds only to partial degrees of completion. The degree of completion is often sufficiently low that phase separation would, and does, occur in a non-agitated plug-flow reactor system. This phase separation is invariably accompanied by deactivation of the noble metal catalyst, either partly or completely.